FIG. 13 schematically shows an example of a conventional surface-mounted antenna. The surface-mounted antenna 1 shown in FIG. 13 is an antenna mounted to a circuit board built into a communication device such as a cellular telephone or the like, and comprises an approximately rectangular parallelepiped dielectric base 2 composed, for example, of a ceramic or resin dielectric member.
A ground electrode 3 is formed over almost the entire surface of the base face 2a of this dielectric base 2, and also, an electric power supplying electrode 4 is formed on the area on the base face 2a where the ground electrode 3 is not formed, with a predetermined spacing between the electric power supplying electrode 4 and the ground electrode 3. This electric power supplying electrode 4 is formed in a manner extended from the base face 2a to the side face 2b of the dielectric base 2.
Further, a first radiating electrode 5 and a second radiating electrode 6 are formed from the upper face 2c to the side face 2d of the dielectric base 2, with a slit S introduced therebetween, and the first radiating electrode 5 and second radiating electrode 6 are both connected to the ground electrode 3.
The surface-mounted antenna 1 shown in FIG. 13 is mounted to the circuit board in a communication device with the base face 2a of the dielectric base 2 toward the circuit board. A rectifying circuit 7 and an electric power supplying circuit 8 are formed on the circuit board, and mounting the surface-mounted antenna 1 to the circuit board as described above connects the electric power supplying electrode 4 to the electric power supplying circuit 8 via the rectifying circuit 7 by conduction.
In the state of the surface-mounted antenna 1 being thus mounted to the circuit board, once electric power is supplied to the electric power supplying electrode 4 from the electric power supplying circuit 8 via the rectifying circuit 7, the supplied electric power is transferred by capacitive coupling from the electric power supplying electrode 4 to the first radiating electrode 5 and second radiating electrode 6, and the first radiating electrode 5 and second radiating electrode 6 resonate based on the electric power so as to transmit and receive radio waves.
Now, the resonance frequency (center frequency) of the first radiating electrode 5 and the resonance frequency (center frequency) of the second radiating electrode 6 are mutually offset such that the frequency band of the radio waves transmitted and received by the first radiating electrode 5 and the frequency band of the radio waves transmitted and received by the second radiating electrode 6 overlap partially. Setting the resonance frequencies of the first radiating electrode 5 and second radiating electrode 6 thus creates a compounded resonating state between the first radiating electrode 5 and second radiating electrode 6, thereby realizing wider bandwidth for the surface-mounted antenna 1.
However, with the surface-mounted antenna 1 configured as described above, the electric current vector A of the first radiating electrode 5 and the electric current vector B of the second radiating electrode 6 shown in FIG. 13 are parallel. Also, the width g of the slit S between the first radiating electrode 5 and second radiating electrode 6 is narrow, in order to reduce the size of the surface-mounted antenna 1. Accordingly, there has been the possibility that the conduction electric current of the first radiating electrode 5 and the conduction electric current of the second radiating electrode 6 would exhibit mutual interference, this mutual interference resulting in a phenomena wherein either one or the other of the first radiating electrode 5 and second radiating electrode 6 would hardly resonate at all, so a stable compound resonating state has not been able to be obtained.
As means for avoiding this, preventing mutual interference of the electric currents of the first radiating electrode 5 and second radiating electrode 6 by widening the gap g between the first radiating electrode 5 and second radiating electrode 6 could be conceived. However, in order to accomplish this, the gap g between the first radiating electrode 5 and second radiating electrode 6 would have to be widened by a great deal, thereby increasing the size of the surface-mounted antenna 1.
Accordingly, the present inventor has proposed in Japanese Patent Application No. 10-326695 a surface-mounted antenna 1 such as shown in FIG. 12 as a surface-mounted antenna wherein a stable compound resonating state of the surface-mounted antenna 1 can be obtained, with greater bandwidth, and also the size can be reduced. Incidentally, this surface-mounted antenna is not publicly known at the time of making the present application, and thus does not constitute conventional art with regard to the present invention.
As shown in FIG. 12, with the surface-mounted antenna 1 according to this proposal, the slit S between the first radiating electrode 5 and the second radiating electrode 6 on the upper face 2c of the dielectric base 2 is formed at an angle to the square sides of the upper face 2c (e.g., at approximately a 45.degree. angle). An open end 5a of the first radiating electrode 5 is formed so as to wrap around to the side face 2e of the dielectric base 2, and an open end 6a of the second radiating electrode 6 is formed on the side face 2d of the dielectric base 2.
Further, formed on the side face 2b of the dielectric base 2 is an electric power supplying electrode 4 serving as a short-circuiting portion linearly extending from the first radiating electrode 5 to the base face 2a, and a short-circuiting portion electrode 10 serving as a short-circuiting portion linearly extending from the second radiating electrode 6 to the base face 2a in the same manner.
The surface-mounted antenna 1 shown in FIG. 12 is mounted to the circuit board of the communication device such that the base face 2a of the dielectric base 2 is toward the circuit board, and the electric power supplying electrode 4 is connected to the electric power supplying circuit 8 via the rectifying circuit 7 on the circuit board.
In such a state of the surface-mounted antenna 1 being mounted to the circuit board, once electric power is supplied to the electric power supplying electrode 4 from the electric power supplying circuit 8 via the rectifying circuit 7, the electric power is directly supplied to the first radiating electrode 5, and is also transferred to the second radiating electrode 6 by electromagnetic field coupling. Thus, the first radiating electrode 5 and the second radiating electrode 6 resonate, and the surface-mounted antenna 1 operates as an antenna.
With the configuration shown in FIG. 12, the first radiating electrode 5 serves as the electric-power-supplying-side radiating electrode to which electric power is directly supplied from the electric power supplying circuit 7, and the second radiating electrode 6 serves as the non-electric-power-supplying-side radiating electrode to which electric power is indirectly supplied from the first radiating electrode 5. Then, with the configuration shown in FIG. 12, as with the surface-mounted antenna 1 shown in FIG. 13, the resonance frequencies of the first radiating electrode 5 and the second radiating electrode 6 are mutually offset such that a compound resonating state can be realized.
With the surface-mounted antenna 1 according to this proposal, in addition to the slit S between the first radiating electrode 5 and the second radiating electrode 6 being formed at an angle to the sides of the upper face 2c as described above, the short-circuiting portions of the first radiating electrode 5 and the second radiating electrode 6 (i.e., the electric power supplying electrode 4 and the short-circuiting portion electrode 10) are both formed on the same side face b, and also the open ends 5a and 6a of the first radiating electrode 5 and the second radiating electrode 6 are respectively formed on mutually differing side faces 2e and 2d so as to avoid the face 2a upon which are formed the above short-circuiting portions 4 and 10.
Due to such a configuration, the electric current vector A of the first radiating electrode 5 and the electric current vector B of the second radiating electrode 6 shown in FIG. 12 are approximately orthogonal, and prevention of mutual interference of the currents of the first radiating electrode 5 and second radiating electrode 6 can be realized effectively without widening the gap g of the slit S between the first radiating electrode 5 and second radiating electrode 6. Accordingly, a stable compound resonating state can be obtained.
In this way, with the surface-mounted antenna 1 shown in FIG. 12, a stable compound resonating state can be obtained without drastically widening the gap g of the slit S between the first radiating electrode 5 and second radiating electrode 6, thereby widening the bandwidth, and also reducing the size.
The rectifying circuit 7 is necessary for operating the surface-mounted antenna 1, so there must always be an area for forming the rectifying circuit 7 as well as the area for forming the surface-mounted antenna 1, on the circuit board for mounting the surface-mounted antenna 1. Thus, the rectifying circuit 7 has impeded improvement in the mounting density of parts on the circuit board.
Also, there is a tendency to use small parts for the parts making up the rectifying circuit 7, in order to reduce the size of the communication device. However, generally, such small parts have poor voltage-tolerance properties, and there is a danger that the parts making up the rectifying circuit 7 cannot withstand a large voltage for suitably exhibiting the properties of the surface-mounted antenna 1, so it has been difficult to supply high electric power to the surface-mounted antenna 1 for suitable operation thereof. Further, as described above, at the time of electric power being supplied to the surface-mounted antenna 1 from the electric power supplying circuit 8 via the rectifying circuit 7, a relatively large conductor loss occurs in the rectifying circuit 7. In this way, not only is it difficult to supply high electric power to the surface-mounted antenna 1 necessary for suitable operation thereof, but also conductor loss occurs at the rectifying circuit 7, so there has been a limit in the improvement in properties of the surface-mounted antenna 1.
Furthermore, the rectifying circuit 7 being thus configured on the circuit board means that there have been various restrictions regarding the configuration of the rectifying circuit 7, such as circuit configuration and parts positioning, etc. That is to say, it has been difficult to configure a desired rectifying circuit 7 at an appropriate position for the surface-mounted antenna 1, leading to the problem that rectification for the surface-mounted antenna 1 is not readily achieved. Accordingly, there has been limited improvement of the return-loss properties (gain properties) of the surface-mounted antenna 1.